This version of the pager is best for use when there are a handful of
typically more static fragments to be paged through, such as a set of tabs.
The fragment of each page the user visits will be kept in memory, though its
view hierarchy may be destroyed when not visible. This can result in using
a significant amount of memory since fragment instances can hold on to an
arbitrary amount of state. For larger sets of pages, consider
FragmentStatePagerAdapter.

When using FragmentPagerAdapter the host ViewPager must have a
valid ID set.